1. Field of the Invention
The subject invention is directed to electrical fuses and, in particular, blade-type fuses as used in vehicular electrical systems.
2. Description of the Prior Art
As in other electrical systems, fuses have been used on vehicular electrical systems to protect the electrical circuits and to insure the safety of operators, passengers and mechanics. For many years, such fuses were of the type generally referred to as cartridge or cylinder fuses wherein a fuse element is housed in a transparent cylinder. The cylinder was closed by end caps that were soldered to the fuse element. The fuse was insertably maintained in a fuse block with the end caps acting as contacts for electrical connection to the respective circuit.
As vehicular electrical systems grew more sophisticated, the number of required fuses also grew. Since the cartridge fuse was somewhat bulky, it was found that a more compact fuse arrangement would be advantageous. The cartridge fuses were also subject to other disadvantages including that they were somewhat difficult to insert and remove from the fuse block and were subject to damage during insertion and removal. Generally a special tool was required for this purpose. Accordingly, there was a need for a fuse of a compact design that was more durable and could be easily inserted and removed from the fuse block.
In response to this need, blade type fuses such as described in U.S. Pat. No. 3,775,724 were developed and introduced. Generally, these fuses were comprised of two flat blades that were secured in a body. The fusible link was located inside the body and soldered to the two blades. Blade fuses were accepted and rapidly became popular because they were compact, durable and could be easily inserted and removed from the fuse block without special tools.
While blade fuses of the prior art were found to be an improvement over cartridge fuses, several disadvantages became apparent. For example, in prior cartridge fuses, the fusible link was made of a zinc alloy that was rolled to a thickness of selected gauge, depending upon the fuse rating. Because this zinc alloy could be rolled within close tolerances, it provided precise fuse links that were found to work well in cartridge fuses. Early blade fuses attempted to utilize similar fusible links of rolled zinc alloy. However, the manner of electrically connecting the rolled fusible links to the blades was a problem in producing commercial quantities of blade fuses.
Early blade fuses attempted to connect the fusible link to the blades by soldering in a manner similar to that of the cartridge fuses. However, soldering required a somewhat enlarged fuse body to accommodate the soldered areas. Also, the soldering techinque was relatively slow, requiring time for the solder to melt and resolidify. Moreover, the fusible link and the blades had to be mechanically held together during the soldering process. For example, in some designs as shown in U.S. Pat. No. 3,775,724, a mechanical interlock between the blades and the fuse link was used.
Another disadvantage of soldring the fusible link to the blades was that the solder connection sometimes interfered with proper operation of the fuse. Specifically, under overcurrent conditions, the solder in some blade fuse designs would melt before the fuse link itself would fail. This would temporarily interrupt the circuit but left the potential that the solder would resolidify in such a manner as to remake electrical contact between the blade and the fusible element, thus maintaining power to the circuit and concealing that the circuit had experienced an overcurrent condition.
Selecting a fusible link having a melting point lower than the melting point of the solder presented another difficulty. Specifically, the fuse elements were difficult to solder to the blades without melting the fuse element in the process.
To avoid these disadvantages, some blade type fuses avoided the problem of connecting the fusible link to the blades by forming the fusible link and the blades from a single piece of metal. An example is shown in U.S. Pat. No. 4,131,869. In these fuses, the fusible link was not rolled to a specified gauge, but was skived down from the same thickness as the blades. This technique was adequate for fuses of sufficiently high ampere ratings where the fusible link did not have to be skived too small. However, at smaller ampere ratings, the fusible link is thinner and more delicate so that it becomes increasingly difficult to skive the fusible link within acceptable tolerances.
In other blade fuses, the fusible link and blades were preassembled by connecting the fusible links to the blades with a heat weld before the blades were secured in the body. These fuses were somewhat difficult to make in that the fusible link and blade assembly had to be connected to the body without breaking the fusible link. Also, this resulted in a blade fuse wherein the body was not molded completely around the blades but left substantially one side of the blades and the fusible link itself uncovered by the body.
Accordingly, it was known that the rolled metal link used in cartridge fuses could be held to very close tolerances. However, no commercially successful mechanism existed in the prior art for rapidly and reliably assembling blade fuses with such relatively delicate links to provide a blade fuse in which the body secured and covered the blades such that the fusible link and blades were unexposed. Accordingly, there was a need for blade fuses with a rolled metal link including a commercially successful method of assembling such a fuse.